In the fabrication of semiconductor devices, there is an on-going need to reduce packaging costs and improve the electrical and thermal performance of the packages. Satisfying these needs is particularly important for "high end" or high performance devices which require packages of tight specifications. For example, VLSI (very large scale integration) and ULSI (ultra large scale integration) semiconductor devices that require very fast signal processing demand packaging materials which have a low dielectric constant and low dissipation factor. One approach to meeting these requirements is by the use of a wiring or circuitized substrate instead of leads, for example a lead frame.
In one view, the dielectric constant of such a substrate should be around 2 or below, and the dissipation factor should be around 0.00025 or below. Substrate materials also need to have structural integrity, including good tensile, compressive and flexural strength properties and good chemical resistance to withstand the many assembly steps and environmental conditions to which the package is subjected. The requirements of a material for a printed circuit board (PCB) are less demanding, in part due to the much larger scale for PCBs, including increased thickness. It should be remembered that PCBs typically bear a relatively large number of already packaged devices. The materials used for PCBs would thus not likely be suitable for device circuitized substrates.
At the same time, it is desirable to keep packaging costs low. Accordingly, high performance substrate materials are preferably able to be fabricated using conventional processes, including molding, extrusion, and coating. Flexibility in manufacturing the material is also desirable, for example, the ability to be drilled, laminated, plated, etched, routed, etc. In surface mount applications, substrate materials should also have very low moisture absorption characteristics. Semiconductor devices made from materials which absorb water later crack or pop open as the absorbed water rapidly expands under conventional high temperature assembly techniques. In addition, it would be highly desirable if the substrate material used had relatively low material density to reduce the mass of the finished product.
Conventional materials used in commercial devices and circuit boards are proving to be less than desirable. Ceramic substrates are expensive and heavy. Also, the commonly used bis-maleimide triazine/epoxy resin blend (abbreviated as BT resin), which has been used in plastic ball grid arrays (PBGAs) has several properties which make it unsuited for high performance packages. BT resins have dielectric constants, E.sub.r, of about 4.3, and dissipation factors of about 0.006. In addition, they absorb as much as 1 wt. % water when immersed in water for 24 hours at a temperature of 24.degree. C. Thus, it would be desirable if a substrate material could be discovered which would meet the packaging requirements of fast, high performance semiconductor devices, while satisfying as many of the foregoing desirable properties as possible.